Oil and gas wells are harsh environments because of the presence of chemically active materials and high temperatures and pressures. Deep wells and development stimulation methods that involve injection of pressurized steam have further raised the operating well temperatures which places even more stress on in-well instrumentation. These harsh conditions create reliability issues for legacy electrical and electronics instrumentation. Optical fiber based instrumentation is more robust and reliable as long as the optical fiber is sealed for both hermeticity and mechanical protection.
One source of failure of an optical fiber is the weaker mechanical connection produced when two strands are joined by fusion splicing. These splices are done by aligning the strand and melting them locally, usually by an arc effect, to fuse them. This method is well known and widely used, but creates a weak mechanical connection that usually has no more than 15% of original fiber axial strength. The fused interphase also leads to a much weaker performance in bending.
U.S. Pat. No. 4,861,133 to Blume et al. and U.S. Pat. No. 5,416,873 to Huebscher et al. illustrate a prior art device for protecting a fused optical fiber joint. In both instances, the protective device is a V shaped clamp receiving the splice that is closed on it such as to prevent it from bending.
Another approach proposed in the U.S. Pat. No. 4,509,820 to Murata et al. is to place the splice in a heat shrunk tube containing a metal rod intended to protect the splice from excessive bends. A drawback of this splice protector is the limited temperature range the splice can tolerate. Heat shrinkable material cannot tolerate very high temperatures which limits the applications of the optical fiber. In addition, the difference of thermal expansion between the metal rod and the optical fiber creates an axial stress on the already mechanically weak joint.
U.S. Pat. No. 5,731,051 to Fahey et al. proposes a sleeve for protecting a fusion splice with a support element made of polymer having a coefficient of thermal expansion which is approximately equal to the coefficient of thermal expansion of the optical fiber. In this fashion, as the fusion splice experiences temperature variations it will expand or shrink at approximately the same rate as the support element, avoiding stresses that would arise otherwise.
U.S. Pat. No. 7,949,289 to Matsuyama et al. proposes a higher temperature material splice protection tube to expand the thermal operating range of the optical fiber, however the range still cannot reach the temperatures encountered in steam stimulated wells which typically vary from 150 degrees C. to 350 degrees C. Similar limitations apply to the splice protector disclosed in the U.S. Pat. No. 5,157,751 to Maas et al.
An optical fiber designed for operations in oil or gas wells must be sealed from chemical contaminants. Typically, this is accomplished by placing the optical fiber in a capillary tube that isolates the optical fiber from the environment. The tube is made from metallic material such as Inconel or stainless steel. Inconel 825 is a specific example of an alloy that can be used for manufacturing the capillary tube. Inconel 825 is considered to be a high performance alloy that offers excellent resistance to heat and corrosion while retaining good mechanical properties such as resistance to stress-corrosion cracking, localized pitting and crevice corrosion.
Challenges arise when a fusion splice covered by a heat shrink splice protector is placed in a capillary protection tube. The splice protector is of larger diameter than the optical fiber and it is heavier, such that it has a tendency to move around in the capillary tube. As such, it is submitted to mechanical vibrations and shocks, thus creating a failure point for the optical fiber.
Therefore, there is a need in the industry to provide a splice protector that is compatible with a capillary tube used in an oil or gas well that alleviates the drawbacks associated with prior art devices.